Melt the butter in a saucepan, stir in the ﬂour and cook for 1 minute, without browning, over low heat. Gradually add the milk and bring to the boil, stirring continuously until the sauce thickens. Reduce the heat and continue stirring for 1–2 minutes to remove all traces of raw ﬂour. Add sugar to taste, then add the cream and rum or brandy. Stir the sauce over the heat for 1–2 minutes, or until hot but not boiling.
Pour into a hot serving jug and serve with the pudding. (The sauce can be made ahead and covered with plastic wrap to prevent a skin from forming.)